1. Field of the Invention
The present invention relates to a magneto-optical recording medium available for magneto-optical recording of data, more particularly, the invention relates to structure and material for composing magnetic recording layers of this magneto-optical recording medium.
2. Description of Related Art
Among those conventional magneto-optical recording media, there is such a magnetic recording layer composed of amorphous alloy thin film made from the combination of heavy rare earth and transition metal, such as TbFeCo or GdDyFeCo, on a transparent glass or plastic substrate having guide tracks. These conventional magnetic films respectively have about 200.degree. C. of Curie temperature. Availing of thermal effect generated by a semiconductor laser beam having about 0.8 .mu.m of wavelength which is condensed to about 1 .mu.m, such conventional magnetic film generates locally inverse magnetic domain for recording and erasing data. This conventional magnetic film can reproduce data by applying those magneto-optical effects like Kerr effect or Faraday effect for example.
In order to promote recording capacity of such a magneto-optical recording medium, it is positively desired to increase recording density by contracting the condensed laser-beam diameter by means of a light source with shorter wavelength. Nevertheless, it is widely known that the shorter the wavelength of light is, the less the magneto-optical effect of the amorphous alloy thin film made from the combination of heavy rare earth and transition metal. Therefore, in order to generate a significant magneto-optical effect by applying a shorter wavelength, such material is composed of amorphous alloy film added with a light rare earth element like praseodymium (Pr), or neodymium (Nd), or cerium (Ce), for example. On the other hand, since the addition of any of these light rare earth elements to the amorphous-alloy film causes the vertical magnetic anisotropy of the magnetic film to be degraded and makes it difficult to stably generate an inverse magnetic domain, a preceding art proposes a means for backing the magneto-optical thin film so that inverse magnetic domain can more stably be retained.
For example, FIG. 1 schematically designates a sectional view of a conventional magneto-optical recording medium disclosed in the Japanese Patent application Laid-Open No. 2-87347 (1990). The reference numeral 1 shown in FIG. 1 designates a transparent substrate accommodating a reflection preventive layer 2, the first and second magnetic layers 13 and 14, and a protection layer 7, which are sequentially superimposed thereon. Of these, the first magnetic layer 13 is substantially composed of an amorphous alloy thin film made from rare earth and transition metal including light rare earth elements like cerium, praseodymium, neodymium, and samarium, or the like, and yet; having a high Curie temperature and weak magnetic coercive force. The second magnetic layer 14 is composed of an amorphous-alloy thin film made from a rare earth and transition metal having low Curie temperature and strong magnetic coercive force.
To record data on the conventional magneto-optical recording medium shown in FIG. 1 initially, such external magnetic field, weaker than the magnetic coercive force of the first magnetic layer 13 and than that of the second magnetic layer 14, is applied and then light is irradiated to the magneto-optical recording medium so as to locally raise the temperature of the recording medium. This causes the magnetism of the second magnetic layer 14 to be lost and instead, an inverse magnetic domain is generated in the first magnetic layer 13, thus permitting a data recording operation to be executed. Next, when the temperature lowers, magnetism of the second magnetic layer 14 is aligned in the direction of magnetism of the first magnetic layer 13, thus permitting a recording operation to be executed on the second magnetic layer 14. As soon as the temperature of the magneto-optical recording medium substantially lowers, due to an exchange couple with the second magnetic layer 14 having strong magnetic coercive force, the inverse magnetic domain of the first magnetic layer 13 is stably retained. Since the first magnetic layer 13 contains light rare earth elements, the first magnetic layer 13 can exert sufficient magneto-optical effect on exposure to light containing such a wavelength shorter than that of infrared semiconductor layer beam, thus providing a satisfactory reproduction effect.
Nevertheless, any of these conventional magneto-optical recording media cannot always generate a fully stable inverse magnetic domain. For example, those two kinds of magnetic-optical recording media shown in Table 1 respectively have a recording layer (shown in FIG. 1) on a plastic disc having 130mm of diameter. The first and second magnetic layers of the recording medium I are respectively composed of TbFeCo film, whereas the recording medium II contains the first magnetic layer in which part of the terbium component is replaced by neodymium. The carrier-wave to noise (C/N) ratio of the reproduced data from a disc of the recording medium III is inferior to that of the recording medium I.
TABLE 1 ______________________________________ Medium I Medium II ______________________________________ First Layer 13 Composition Tb.sub.16 Fe.sub.70 Co.sub.14 Tb.sub.8 Nd.sub.8 Fe.sub.70 Co.sub.14 Second Layer 14 Composition Tb.sub.29 Fe.sub.61 Co.sub.10 Tb.sub.29 Fe.sub.61 Co.sub.10 First Layer 13 Film Thickness 0.025 .mu.m 0.025 .mu.m Second Layer 14 Film 0.025 .mu.m 0.025 .mu.m Thickness First Layer 13 Saturation 280 emu/cm.sup.3 360 emu/cm.sup.3 Magnetization Second Layer 14 saturation 130 emu/cm.sup.3 130 emu/cm.sup.3 Magnetization C/N Ratio 48.1 dB 37.5 dB ______________________________________
After visually checking the pit-form inverse magnetic domain recorded on those tested media with a polarized microscope, inventors witnessed that the recording medium I contained smooth elliptic pit-form, whereas the other recording medium II merely contained such pits devoid of smooth shape, and yet, some pits even contained those domains without being inverse-magnetized. Probably, this is because the saturation magnetization is increased in the first magnetic layer of the recording medium II as a result of the addition of the Nd component, thus causing the floating magnetic field to increase in the layer. In consequence, the first magnetic layer can not fully and stably generate and retain the inverse magnetic domain.
More particularly, in order to provide a substantial magneto-optical effect using a short wavelength, light rare earth elements are added to such a conventional magneto-optical recording medium, however, any of these conventional magneto-optical recording media cannot always constantly achieve satisfactory reproduction characteristic.